[Ended]

The surface is the data. E.g. the ridership for Lafayette Street is the upper line of the green area, minus the lower line of the green area. This is called an area chart and is used to show cumulative data and how each part contributes to the whole. In this case it shows the total NYC ridership increasing from ~7000 to ~18000 (units are unclear!), while e.g. the Central Park West ridership has remained approximately constant.

[Someone]

My first thought when seeing that chart was "How to lie with statistics".

Stacking those graphs in that way only is valid if there are no double-countings in the data. New York is large, but I think/guess (I know little of street layout in NY, but looking at coordinates, Layafette street at Astor plaza and Second Avenue at 7th Street seem fairly close to each other) that is unlikely.